USEFUL INFORMATION
AGM (Absorbed Glass Mat) Batteries
Are spill-proof and deep-cycle, BUT cannot be charged at anything like the same rate as un-sealed wet-cell batteries - a major disadvantage on most sailing boats.
Types - the output voltage of an alternator is usually controlled by a simple, solid-state electronic, internal regulator that limits the alternator output voltage either at the output terminal (machine-sensed) or at the ‘sense’ terminal (remote-sensed) which is connected to some other point - usually the battery. A remote-sensed alternator becomes machine-sensed if these two terminals are connected together. WARNING - If the sense terminal is not correctly connected the regulator or the diodes inside the alternator will self-destruct! Typically, the regulator is designed to limit the voltage (at the sense point) to a nominal 14.4V, but a typical specification will quote 14.2V ±0.4V.
Output - the alternator’s quoted rating (e.g. 55 Amp) is a maximum, developed at high speed and a lower voltage than is required during battery charging. When charging, the average output current will only be about 60% of the rating (e.g. 33A) even with the engine running at high revs. Some of this current will be taken by anything else in use and only the remainder is available to charge the battery. To recharge a 110 Ah battery from 25% to 80% with a 55A alternator will take 2 hours or more with the engine running hard (55% of 110 = 60 Ah divided by 30A allowing only 3A for instruments etc.) Actual alternator output can be much improved by the use of a ‘smart’ regulator.
The usable capacity of a battery (bank) will be about half of the nominal rating for two reasons:
1. A conventional regulator is unable to achieve much more than about an 75% charge even with prolonged charging (a ‘smart’ charger or regulator can achieve 95%+ ).
2. Even ‘deep-cycle’ batteries won’t last if repeatedly discharged much below 25%,
Thus 75% - 25% = 50%, but 95% - 25% = 70% so a ‘smart’ regulator or charger increases the usable battery capacity by 40%.
Because batteries are always referred to as 12-volt (or 6V or 24V), it is often assumed that the normal voltage is 12V. In fact a ‘12V’ lead-acid battery only producing 12.0V is either almost flat or is delivering a large current (under a heavy load). In fact, a healthy ‘12V’ battery, when not being charged, should always show 12.2 - 12.8V.
Very conveniently, the relationship between a battery’s state of charge and its voltage is linear (10% per 0.10V) between about 90% (12.70V) and 20% (12.0V). However, when a battery has been on charge, even if it’s not fully charged, the voltage will be up around 13.8V. This will slowly drop to around 13.6V over the next few hours, but even if left overnight it shouldn’t drop below 13.0V unless the battery was only partly charged or is on the way out. But, as soon as a reasonable sized load is switched on, the voltage drops quite quickly until it equates to the battery’s actual state of charge. Thereafter, the rate of voltage change (under constant load) becomes linear. Thus, in practise, the voltage of a freshly charged battery is a poor guide to its state of charge until some of the charge has been used. With a conventional internal alternator regulator, the best guide to when charging is as complete as it’s going to be, is when the voltage at the battery remains constant for ten minutes or so at the regulator’s maximum of around 14.4V. If it never reaches 14.2V, first check both sides of the charging circuit for poor connections. If that doesn’t solve the problem, have the alternator checked over and the regulator changed if necessary. 14.4V is the point beyond which gassing starts to occur under prolonged charging (continuous motoring), but the closer to it that the regulator gets, the faster and more complete the charge will be. For more details with a diagram showing a typical Voltage -v-State of Charge curve, go to the Design Principles page
Choice & Care of Batteries
Despite constant advances in battery technology, all batteries are a compromise in design. At opposite ends of the spectrum are high current (automotive or engine-starting) and deep cycle types. High current batteries are designed to be capable of producing a very high current for a short period (several hundred amps for a maximum of 30 seconds), but they don’t take kindly to being deeply discharged (much below ½ charge). At the other extreme, deep cycle batteries are designed to happily deliver most (80%+) of their charge, but over a period of several hours. No (lead-acid) battery takes kindly to being completely discharged – it always results in some loss of capacity and leaving a battery flat is guaranteed to wreck it. The best way to make a battery last for years is to charge it up as soon as possible, never discharge it below 40% (high current), 30% (marine/leisure) or 20% (traction). Never ever try to start an engine with anything other than a high-current battery that is at least 50% charged. TLC for boat batteries includes taking care of them during winter – it’s not enough to give them a bit of a charge every month or two with a cheap mains charger for the reasons explained below.
Service Batteries - Automotive batteries are only suitable if you are prepared to accept their 10% lower usable capacity (50% of nominal against 55% for a ‘leisure’ or ‘marine’ battery) and greater weight. Marine/Leisure batteries are designed for this purpose and offer the best compromise between usable capacity, weight, and life. There is a bewildering array on offer - generally speaking you get what you pay for.
Engine Start Battery – a good quality heavy-duty sealed-for-life automotive battery of a rating (cranking amps not amp-hours) appropriate to your engine is ideal. Such a battery, with a 3 year warranty, is designed for a minimum of 1,000 cold starts. If you can’t easily determine the cold-cranking current for your engine, a reasonable guide will be a battery for a diesel car or van with the same or larger cylinder capacity (engine capacity divided by number of cylinders) as your engine (e.g. Beta BD722 719cc divided by 3 cylinders = 240cc per cylinder. Lister-Petter Alpha 20, 930cc ÷ 2 cylinders = 465cc). Most automotive diesel engines are 4 cylinder units of 1.4L (350cc per cylinder) or more, so in the Beta example, the smallest diesel battery you are likely to find will be more than adequate. In the case of the Lister, 4 x 465 = 1,860cc so a battery for a 1.9 or 2 litre diesel vehicle would be suitable. The other considerations when choosing a battery are
1. Type of terminal (tapered round post or square lug).
2. Orientation of terminals (Positive on Left or Right with the terminals nearest you).
3. Physical dimensions - don’t forget overall height including posts, clamps and space needed for the cables to reach the posts.
You don’t want to find that you’ve bought a battery with the wrong type of terminal or the cables need to cross over but aren’t long enough, or it won’t fit in the available space. The engine start battery should be wired direct to the starter motor (not always the case) so that the initial current surge of several hundred amps doesn’t go through a battery master switch designed to take a maximum of say 60A! Provided this cable is short (as it should be in view of the current it has to carry) it represents no more of a risk of shorting than does a cable to a battery master switch. A technically more elegant (but expensive) solution is remotely operated isolation relays/solenoids/switches.
When a battery is being charged, the electrical current causes an electro-chemical reaction; this reaction is reversible to produce electrical power. However, like all chemical reactions it is time dependant, and it also takes time for the charge to penetrate into the depth of the active material. Once the charging voltage goes over 14.4V, part of the charging current is dissipated by splitting the water in the battery electrolyte (dilute sulphuric acid) into hydrogen and oxygen gases. This mixture is EXTREMELY EXPLOSIVE - battery compartments must be well ventilated. It is normal for there to be a small loss of water during charging. Sealed batteries employ a catalyst to safely convert the gases back to water – however, they can only cope with a limited amount so excessive ‘gassing’ must be avoided. Old technology batteries need regular topping up with distilled water to make good the lost water. All batteries used on a boat should be spill-proof because mixing battery acid with sea-water produces chlorine gas - poisonous and highly corrosive.
Most (but not all) inexpensive mains chargers are pretty crude devices designed to put enough charge back into a vehicle battery to start an engine, but without running the risk of over overcharging if left on for several days. The charger is simply a step-down transformer, a simple rectifier and a cheap ammeter (amp-meter). The transformer is wound so that the maximum DC output is less than 14V. The actual charging current only reaches the rated maximum below about 10V and is normally about a quarter of this or less. Such a charger is incapable of giving anything remotely like a full charge, even after several days. Switch-Mode chargers are considerably more expensive but, by using much more sophisticated electronics to control both voltage and current, are capable of producing their rated maximum output during most of the charge cycle. Moreover, most switch-mode chargers employ a ‘smart’ charging regime which will relatively quickly bring a battery to a more or less full charge (around 95%) and then switch over to a ‘float’ mode which keeps the battery fully-charged without significant gassing.
Many panels are capable (in direct sunlight) of producing a voltage in excess of the maximum battery charging voltage of 14.4V. Conversely, in low-light conditions the cells don’t generate enough voltage to prevent the battery discharging back through the panels unless a diode (which acts as a one-way valve) is incorporated – most panels have one built-in, but check. To prevent solar panels from overcharging and wrecking your battery, a regulator is required. ‘Shunt regulators’ are effectively electronic safety-valves that dump the power from the solar cell into a suitable sized load (the ‘shunt’) when the output voltage rises above about 14.4V. The shunt must be matched to the panel output – if it’s too small it will burn out. There are sophisticated (= expensive) ‘smart’ solar panel regulators which will maximise the charge from the panel and also simple regulators which isolate the panel once the charge voltage hits a pre-set limit (14.4V) and don’t reconnect until the battery voltage drops to say 13.4V (either the panel has been off overnight or a load is now connected).
Twin Battery Installations
It is usual today for all but the smallest boats to have twin batteries. The basic idea is safety-related - a separate engine start battery means that there should always be plenty of battery power to start the engine, even if the other, service or domestic, battery has been completed flattened by accident. It is common to use a 1-2-Both-Off rotary battery master switch so that that in an emergency either battery can be used to start the engine or power the VHF. The start battery should always be wired directly to the starter motor solenoid and the rotary switch (controlling power to everything else and from the alternator) doesn’t carry the very large (several hundred amps) starting current for which it is unlikely to be rated. The convention is that the engine-start battery is wired to Position ’1’ (only for emergencies) and Position 2 to the service (or domestic) battery. ‘Both’ should only be used when charging without split-charge diodes. A common mistake is to put the switch to ‘Both’ when one battery is well down; all that happens is that it drags the other battery down so that they both end up equally flat. When not charging, the switch should be in position ‘2’. On larger boats, it is common to have two or more batteries connected together as a ‘bank’ behaving as a single battery. Single batteries much larger than about 100Ah are too heavy to lift by hand, and it’s often easier to find space for two separate batteries than one big one. It’s important that both batteries in such a pair are of the same type and capacity, and preferably are identical.
Rotary change-over switches (1-2-Both-Off) must make-before-break, otherwise all power will be momentarily lost when changing between batteries. When on BOTH, remember that the engine start current of several hundred amps will go through the switch, which may not be rated for such a heavy load. See also comments about wiring under starter batteries and starting and charging regime for twin battery installations.
Also known as blocking diodes or a diode block. Acts as an electronic one-way valve between the alternator and twin batteries to enable the alternator to charge both batteries without current being able to flow from one battery to the other. Thus the start battery will remain fully charged when the service battery is in use. As there is a voltage drop across the diodes, it is essential for the alternator to be remote-sensed (with ‘Sense’ connected to the service battery positive terminal) rather than a machine-sensed. Otherwise, as the alternator regulates itself to a maximum of 14.4V at its output terminal, there will only be about 13.4V at the batteries which will thus never get anything like fully charged.
Starting and charging regime
For twin battery installations with a 1-2-Both battery switch and no split-charge diodes. (With split-charge diodes leave the switch in ‘2’ all the time.)
You want an operating procedure that always keeps the engine battery at the fullest possible charge at all times, thus:
Position ‘1’ is only used in an emergency when there is not enough power in the service battery to operate the VHF.
Always start the engine with the switch at ‘2’, thus isolating the rest of the electrics from the start battery - which may temporarily drop well below 11V - and maintaining power at 12V+ to the instruments. As the engine warms up, switch to ‘Both’ so that both batteries are charged. As soon as the engine is stopped, turn the switch to ‘2’ so that the start battery doesn’t get discharged.
‘Smart’ Voltage Regulators
These effectively by-pass the alternator’s internal ‘dumb’ regulator and provide a much more effective charging regime. Typically they reduce battery-charging time by a factor of 2 or 3 while also achieving a much fuller charge, in effect nearly doubling the usable battery capacity (see Battery Capacity above). Nonetheless, the alternator has to be rotating reasonably fast to produce its maximum output. Visit http://www.adverc.co.uk or http://www.sterling-power.com for information on two competing ranges of battery management systems.
This serves much the same purpose as blocking diodes, but offers (at a price) considerable benefits: Connected to the alternator output, it stays switched to the engine battery until its voltage indicates it is ‘fully’ charged. The relay then switches charging to the service battery (bank). Alternator regulation, whether internal or via a ‘smart’ regulator is always from whichever battery (bank) is being charged, thus eliminating the possibility of over-charging the engine battery while optimising charging.
The Conventional Test of battery capacity (complete discharge from ‘fully-charged’ over an expected 20 hours through a known load) has two major flaws: 1, only after extended charging will a conventional alternator achieve much more than an 80% charge and ordinary (i.e. not switch-mode) mains chargers even less. 2, as anyone who has accidentally left the car lights on all day knows, completely discharging a battery will almost certainly bring about its early failure. A hydrometer, the only accurate means of measuring the state of charge, can’t be used on modern sealed batteries. This test is even less accurate if, as is usually suggested, the load is made up of 12V lamps and the formula used is Capacity = Hours x Total Watts / Volts. The two ‘constants’ (Watts and Volts) are anything but constant: the voltage will drop from around 13½V to 10½V (a change of 25% ) and as it does so, the lamps run cooler and their resistance decreases but not in direct proportion.
An Alternative Method exploits the fact that, apart from the initial and final stages of discharge, battery voltage (under constant load) declines at a linear rate of 0.1V per 10% of battery capacity. This method is quicker, harmless to the battery, accurate, and checks the initial state of charge. In theory, the fully charged and discharged voltages are 12.8V and 11.8V respectively for an ‘equalised’ battery (left long enough for the voltage to have completely stabilised). This doesn’t happen in reality, but the voltage of well-charged battery under a suitable constant load will fall at a constant rate once the first 5-10% of charge has been used. By measuring the rate of change of battery voltage under such conditions, it is fairly simple to calculate the actual and usable capacities and the approximate initial state of charge.
Test Equipment & Preparations
For the test, one needs a wrist-watch, a VoltWatch, and a resistive test load (lamps) roughly equivalent to the 20-hour rate. Test load (Watts) = nominal battery Ah x 12V / 20 hours, e.g. for a 110AHr battery, a test load of 66W (110 x 12 / 20) is required. The test load doesn’t have to be exactly that calculated, so long as we know what it is – in our example, a 60W or 75W car headlight would do. The test load will dissipate a lot of energy as heat – so don’t leave a headlight aimed at the furnishings! Start by ‘fully charging’ the battery (bank) under your normal conditions and then let it ‘settle’ for several hours without being used.
Doing the Test
Stage 1. Note the time (T1) the test load is switched on . Allow the battery to stabilise with the load on until the ‘F’ indicator goes out (12.62V); note the time (T2).
Stage 2. Leave the load switched on until amber ‘R’ first lights (12.20V), note the time (T3). Disconnect the test load and put the battery back on charge.
Calculating the results
Actual Capacity = Average discharge current x Time / percentage Discharge
Average current = Load (in Watts) / Average Voltage = W / V2 – ½ (V2 – V3) Discharge % = 100 x Voltage drop. [1.00V between theoretical full charge and flat voltages]
Capacity = {W / V2 – ½ (V2 – V3)} x (T3 - T2) hours / (V2 – V3)
Example: T2 to T3 took 8 hours 20 minutes, while the voltage dropped from 12.62V (V2) to 12.20V (V3).
Actual Capacity = {60 / 12.62 – ½ (12.62 – 12.20)} x 8.33 / (12.62 – 12.20) = {60 / 12.41} x 8.33 / 0.42 = 96AHr (88% of nominal)
Initial State of Charge = State of Charge at T2 + percentage discharge during Stage 1
State of Charge at T2 = 100% – 100 (12.80 - V2) [12.80V is the theoretical fully-charged voltage]
Stage 1 discharge (Amp Hrs) (T2 – T1) x Load / (12.80 – ½ (12.80 - V2)) % discharge = Stage 1 discharge / Actual capacity
Example: State of Charge at T2 = 100 – 100 (12.80 – 12.62) = 82%
Average current T1 to T2 = 60/ (12.80 – ½ (12.80 – 12.62)) = 60/12.71 = 4.72A
In this case Stage 1 (T1 to T2) took 1hr 15mins
Stage 1 discharge = Average current x time = 4.72 x 1.25 = 5.9 AHr
Stage 1 % discharge = 100 x 5.9 / 96 = 6%
Initial State of Charge = 88% (82+6) – very good, 75-80% is more typical.
Conclusions
As a battery shouldn’t be discharged much below 30% (12.10V) and normal charging gives an 88% charge, this battery’s usable capacity is about 55hr (58% of 96Ahr), half the 110AHr rating and a lot less than is often presumed.
